Recently, closed-draw-type papermaking machines having no open draw section have been developed to speed up papermaking operation.
A typical closed draw papermaking machine is shown in FIG. 5.
A wet paper web WW, shown in a broken line in the figure, is transferred from the right to the left, being supported by press felts PF1, PF2, a wet paper web transfer belt TB, and a dryer fabric DF. The press felts PF1, PF2, the wet paper web transfer belt TB, and the dryer fabric DF are, as is generally known, endless belts and held by guide rollers GR. A shoe PS has a concave bottom which conforms to the press roll PR. The shoe PS and the press roll PR, with a shoe press belt SB in-between, compose a press part PP.
The wet paper web WW, after traveling through a wire section and a first press part (not shown in the figure), is transferred from the press felt 1 onto the press felt 2. It is then transferred to the press part PP by the press felt 2, where the wet paper web WW, sandwiched between the press felt PF2 and the wet paper web transfer belt TB, is compressed by the shoe PS and the press roll PR with the shoe press belt SB in-between.
The press felt PF2 has a high water permeability, whereas the wet paper web transfer belt TB has a very low permeability. Accordingly, within the press part PP, the water contained in the wet paper web WW moves into the press felt PF2. The press felt PF2, the wet paper web WW, and the wet paper web transfer belt TB, rapidly released from the compression upon leaving the press part PP, back to their uncompressed state. This expansion of the volume, coupled with the capillary phenomenon of the pulp fibers composing the wet paper web, causes rewetting in which some of the water within the press felt PF2 backs to the wet paper web WW. However, the wet paper web transfer belt TB does not hold water therein because of its low permeability. Thus, the wet paper web transfer belt TB causes little or no rewetting and hence facilitates dewatering of the wet paper web. The wet paper web WW, after exiting the press part PP, is transferred by the wet paper web transfer belt TB. The wet paper web WW is then sucked onto a suction roll SR and a dryer fabric DF transfers it to a drying section.
The wet paper web transfer belt TB is required to have a function to allow smooth detachment (paper release) of the wet paper web WW when it transfers it to the next process.
Unexamined Japanese Patent Publication No. 89990/2001 discloses an example of the wet paper web transfer belt with such a function, in which a paper side layer comprises a high molecular weight elastic section and a fiber body, either of which is made from a hydrophobic material. According to this invention, the hydrophobic material breaks a thin water film formed between the wet paper web and the wet paper web transfer belt, after the wet paper web travels out of the press section, which enables smooth transfer of the wet paper web to the next process.
On the other hand, another important consideration has become increasingly recognized that the wet paper web transfer belt should be capable of having the wet paper web attached securely thereon at the exit of the press section.
While the wet paper web transfer belt disclosed in the Unexamined Patent Publication No. 89990/2001 is capable of having the wet paper web detached smoothly therefrom, it has a problem that the paper web sometimes gets torn during and at the time of transferring to the next process, because the thin water film between the wet paper web and the belt is broken at the exit of the press section, where the wet paper web does not securely stick to the surface of the belt.
With the object to solve this problem, the inventor of the present invention proposed, in Unexamined Japanese Patent Publication No. 277971/2004, a wet paper web transfer belt with a paper side layer which comprises a high molecular weight elastic section and a fiber body, a part of the fiber body being exposed on the surface of the belt. According to this invention, since hydrophilic fibers exposed on the surface of the paper side layer holds the water removed from the wet paper web, the belt is provided with a balanced combination of functions to transfer the wet paper web attached thereon and to allow smooth detachment of the paper web when transferring it to the next process.
The machine side layer of the wet paper web transfer belt of the Unexamined Patent Publication No. 277971/2004 is a batt layer made of staple fibers. However, since this type of machine side layers are subjected to high frictional resistance, the batt fibers on the surface of the machine side layer tend to get worn in a shorter period of time due to friction, as the wet paper web transfer belt runs around the press section and guide rollers of the papermaking machine; a problem recognized by those skilled in the art.
Wear-resistant property has been required to machine side layers, because the machine side layer of a wet paper web transfer belt acts as a cushion of the belt and serves to maintain dimensional stability of the belt by protecting a base body of the belt.
Therefore, it is an object of the present invention to provide a wet paper web transfer belt having a low-friction and less-wearing machine side layer.